I suspect that the overwhelming majority of cases that warrant inline assembly would not benefit if it was instead a function call out to something you've linked in. That is, most inline assembly is a small snippet of optimized code that necessarily exists in the function you're optimizing.
Inline from C libraries won't -- as far as I'm aware -- inline into a Rust function. Inline is a compile-time hint, not a link-time behavior. In theory this could be changed in LLVM, but it seems really unlikely to me. The function exposed from C (with the inline asm) can be naked, though, assuming you are hygienic.
Yeah, as always, when we're considering stabilizing or removing something, we talk to who is using the feature and in what way, and these kinds of issues surface.
